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# The mole and Avogadro's number

AP.Chem:
SPQ‑1 (EU)
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SPQ‑1.A (LO)
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SPQ‑1.A.1 (EK)
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SPQ‑1.A.2 (EK)
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SPQ‑1.A.3 (EK)
One mole of a substance is equal to 6.022 × 10²³ units of that substance (such as atoms, molecules, or ions). The number 6.022 × 10²³ is known as Avogadro's number or Avogadro's constant. The concept of the mole can be used to convert between mass and number of particles.. Created by Sal Khan.

## Want to join the conversation?

• what is the relationship between mole and amu?
1mole of an element is equal to the amu no.of grams but is there another relationship also what is atomic mass and atomic number
THANK U! •   Hi, this is the way I came to understand the concept of a mole.

Recall that,
1 amu = 1.6605 x 10^(-27) kg and
1 mole = 6.02 x 10^23.

Therefore,
1 mole of amus
= 1 mole of 1.6605 x 10^(-27) kg
= 6.02x10^23 of 1.6605 x 10^(-27) kg
= 6.02x10^23 x 1.6605 x 10^(-27) kg ("of" means "x")
= 0.001kg
= 1g

(Please note: Some of the numbers above have been approximated and rounded but for simplicity I've left that stuff out.)

I think that 1 mole was chosen to be 6.02x10^23 so that when it was multiplied by the amu it would give a nice number like 1g (shown above). I also think they chose to invent the mole because the mass of a proton or neutron is such a small number that they needed a way to descibe the mass in quantities that we can work with.

I've included an example below showing the same concept to the above example but in a more familiar context. I suggest comparing line-by-line with the above example to see the similarities.

Example 1: Let's determine the mass of 1 dozen eggs.

1 egg mass = 60g and
1 dozen = 12.

Therefore,
1 dozen of egg masses
= 1 dozen of 60g
= 12 of 60g
= 12 x 60g
= 720g

Example 2: Show that 1 mole of 12C is 12g.

Recall that,
1 amu = 1.6605 x 10^(-27) kg and
1 mole = 6.02 x 10^23.
12C = 12 amu

Therefore,
1 mole of 12C
= 1 mole of 12 amu
= 1 mole of 12 x 1.6605 x 10^(-27) kg
= 6.02x10^23 of 12 x 1.6605 x 10^(-27) kg
= 6.02x10^23 x 12 x 1.6605 x 10^(-27) kg
= 0.012 kg
= 12g

Generally when working with Avogadro's number and the mass of a proton or neutron, you wouldn't substitute the values in. I was just trying to show why 1 mole of amus equals 1g.

Anyway, I hope this helps.

Mars.
• • So if my understanding of a Mole is correct, basically it is just a tool that was create so that amu could be associated in more simple terms such as a gram?
(1 vote) •  My understanding is this... the amu of any specific atom * a mole = the mass of that atom in grams. The amu number value of the specific atom should equal its mass in grams, even after the conversion. :) Hopefully that helps!
• what was sal telling about relation between 13 almunium and avagadro number when he stooped its making all this confusing? • I agree completely with Just Keith's answer of the over complication of this video along with his great detail on the background and mathematics of the mole and atomic mass unit.

I would just like to add maybe an even more basic explanation that I would employ to start with: when going to the grocery store to shop for you and your family, you would not pick up eggs individually anymore, or say 27 mL of milk or a specific amount of grains of salt (say 5000 grains), you would get say a donzen eggs, a litre/gallon of milk and specific mass/weight in gram/ounce of salt depending upon which unit of measure you use, this being more convenient and manageable number(s) when shopping. The last item, the salt, it would not be feasible to count out grains or even to determine the grain size being consistent, but would be more like the problems encountered for the much smaller atom/compound/molecule.

Now, for Chemistry when dealing with atoms, compounds or particles of elements, the 'convenient number' is the mole, which makes the use of the atomic mass numbers on the Periodic table manageable and much easier to use both in theoretical calculations and practical experimentation with chemicals. Then I would go into the more specific and accurate details given by Just Keith to explain the concept thoroughly.

I hope this story/example helps out as a starting point for what can be a very difficult concept depending on how it is approached.
• • I was always told that mass, the SI unit for which is the kilogram, was just a measure of how much matter is in an object. However, the mole is basically measuring just that; the amount of amus, or matter in an object. Yet, both the mole and the kilogram are SI units. What is the difference between the two? • So, a Mole is just basically a convenient way to convert between grams and atomic mass units? • • i am confused......
In the starting of the video it is mentioned that 1 mole is = 1 gram
but later he said that 1 mole of iron = 56 grams as atomic mass of iron is 56 so why he said that 1 mole is = 1 gram?????  